The present invention relates to a bilayer adhesive system for use in natural rubber adhesive tapes and particularly for use in a reinforced strapping tape for use in the appliance packaging industry.
The suitability of any adhesive tape for use in a particular application is a function of several factors. Properties of the adhesive such as peel strength, tensile, tack and the like are important factors, but other properties of the tape and its construction also affect its usefulness including the flexibility, softness and elastomeric character of the adhesive layer.
Reinforced, pressure sensitive adhesive tapes (PSAT) have been used in the appliance packaging industry for many years. Typically, such tapes are constructed from a backing substrate having, on its surface, a layer of reinforcing strands and a pressure sensitive adhesive (PSA), particularly a natural rubber-based pressure sensitive adhesive (NRPSA). These reinforced, natural rubber adhesive tapes can be applied to a variety of appliance surfaces (adherends) with little or no surface preparation. However, one important disadvantage of these products in particular and natural rubber pressure sensitive adhesive tapes in general is their relatively high manufacturing cost. Due to the relatively high coating weights associated with reinforced, strapping tapes with NRPSAs and because natural rubber adhesives must be cast from a solvent at relatively low solids, the adhesive must be coated at slow line speeds (e.g., about 150 fpm) to avoid breaching the lower explosive level (LEL) of solvent vapor in the drying ovens of typical coating lines. While one solution to this problem is to coat the natural rubber adhesive in multiple layers in tandem, this increases processing equipment costs, e.g. multiple coating heads and/or additional dryer capacity.
Attempts have been made to construct tapes using hot melt pressure sensitive adhesives (HMPSA). Typically, reinforced strapping tapes are constructed by applying to a backing support two adhesive layers, e.g., an acrylic laminating pressure sensitive adhesive layer and a top hot melt pressure sensitive layer. The acrylic layer, which is typically deposited from an emulsion, bonds the reinforcing strands to the backing substrate. The hot melt pressure sensitive adhesive is applied as a top coat which fills the spaces between the reinforcing strands and ensures an uninterrupted adhesive layer atop the strands. While the raw material cost of the reinforced hot melt adhesive is, on average, higher than that of the natural rubber-based reinforced pressure sensitive adhesive, less of it is needed (typically 30 lbs. per 3000 ft.sup.2 vs. 48 lbs.). Furthermore, it can be applied in tandem with the acrylic emulsion adhesive at relatively high line speeds, when compared to solvent cast natural rubber adhesives (typically 600 fpm vs. 150 fpm) at a lower processing cost which translates into a lower total cost than the natural rubber-based tape.
One disadvantage with hot melt pressure sensitive adhesive tape is that it requires greater care to apply to the adherend, i.e., it requires a dust-free, grease-free surface and careful rub-down. Also, the hot melt pressure sensitive adhesive does not exhibit consistently high wet-tack and adhesion to and transfer-free removal from all of the adherends encountered in the appliance packaging industry.
Accordingly, there is a need in the industry for a pressure sensitive adhesive tape and, more particularly, for a reinforced strapping tape which exhibits the adhesion wet-tack characteristics of natural rubber adhesives and which can be produced at a lower total cost.